Manufacture of water finished board having an insolubilized protein-mineral coating t hereon



Dec. 28, 1954 s. w. TROSSET, JR.. EIAL 2,698,2 9

MANUFACTURE OF WATER FINISHED BOARD HAVING AN INSOLUBILIZED PROTEIN-MINERAL COATING THEREQN Filed May 27, 1950 INVEN TORS. STANLEY M 7TeossEr, Je,

mrduuufw ATTORNEYS. b

United States Patent 2,698,259 MANUFACTURE or WATER FINISHED BOARD HAVING AfN INSOLUBILI-ZED PROTEIN-MIN- ERAL COATING THEREON Stanley W. Tro'sset, .Ir., Deer' Park, and Harry C. Fisher and Simeon Richard Hoimes, Cincinnati, Ohio, assignors to The Gardner Board and Carton (30., Middle- Our invention relates to the coating of paper or paperboard with mineral substances in a suitable binder for the purpose of imparting appearance and printing qualities which the uncoated prodnc't does not possess. Our coated paperboard is converted into folding cartons, boxes, signs, etc. Casein has hitherto been suggested as a binder for mineral coatings on paper and board; and it has been known that casein as a binder can be insolubilized by water solutions of metallic salts and some acids. These solutions are acidic in nature. Paper makers alum, aluminum sulphate, is effective, as is sulphuric acid. This is true for proteins iiigeneral, such as protein from soya beans. fromanimal glue, etc., which sometimes are used to replace part of the casein for reasons of economy or for special effects. For example, if a mineral coating composition having casein as a binder has been imposed and smoothed, as by brushes or rollers, on the surface of paper or board, and dried, then the moistening of that surface with a water solution of alum followed by a drying of it insolubilizes the casein and Waterproofs the coated surface to the extent of making it at least resistant to wet rubbing. Such products have also been calendered for improvement in their surface characteristics; but attempts to give them a water finish have been disappointing. If a water finishing procedure is applied before the insolubilizing of the coating, the coating becomes softened during the operation and tends to pick off on the calender rolls with consequent impairment of the finish. Moreover. thejsubsequent application of a water solution of acidic' substance for the insolubilizing of the casein will impair whatever measure of surface smoothness is attained in the water finishing attempt. On the other hand, if a water finishing procedure is applied after the insolubilizing step described above, there will be insufficient softening of the binder to enable. the attaining of asatisfactory water finish.

A primary obiect of our invention is the provision of an inexpensive but hiahlv effective DIQcedure for the formation of a wafer finished insolubilized casein coating on paper or board.

Other important objects of our invention have to do ivithdthe provision of improved casein coated paper or oar These and other obiects of our invention, which Will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, We accomplish by that procedure and in that product of which we shall now describe an exemplary embodiment; Reference is made to the accompanying drawing wherein is shown in diagrammatic form the elements of. an apparatus with which ourpr'ocedure may be carried on.

Briefly, in the practice of our invention, we have found that under certain circumstances hereinafter outlined, it is possible to manufacture coated paper or board in which the coating binder is an insolubilized casein, resistant to moisture and wet rubbing, the product having at the same time all of the desirable qualities of a water finished coated sheet, by a procedure which involves simultaneous water finishing and insolubilizing.

In Patent No. 2,419,207, issued- April 22, 1947, in the name of Harry C. Fisher, one of the applicants herein, there is described a process for the manufacture or coatedpaper and board in which the felted web, after drying on themachine, has its surface or surfaces subjected to a so-called plasticizing treatment. This treat- Patented Dec. 28, 1954 ice ment involves passing the web through a breaker stack of calenders and imposing upon one or both surfaces, by means of Water boxes on the calender rolls, a plasticizing solution, the surface characteristics of the web being thus modified to a condition peculiarly favorable for the acceptmice of a thin imprinted coating of mineral coating substance in a suitable binder. The surface preparation is the combined result of mechanical work exerted by the calender rolls and conditions brought about by the plasticizing solution itself; and the mineral coating should be imposed upon the plasticized surface or surfaces during a persistence of the plasticizing effect. When this is done, a coating suspension consisting of suitable finely divided mineral coating substances in a solution of a hinder, the mixture having a printable consistency or body, may be imprinted upon the web in an exceedingly thin layer, the web then being dried and recalen'dered. Coatings produced in this way are inexpensive because of the minimum quantity of materials involved and because they are produced coincident with the paper or board making process; and the coated paper or board is of exceptional quality and has great appearance value.

The plasticizing solution is, as set forth in the above mentioned patent, usually a water solution of starch or polyvinyl alcohol, or both. In the practice of the present invention, we employ the same types of plasticizing solutions and in the same way. The coating compositions consist essentially in a water suspension of suitable, usually mineral, coating substances of sufficient opacity and desirable color, and abinder which not only assists in the dispersion of the coating substances in the Water vehicle, but also binds the coating substances together and to the web surfaces when the imprinted coating has been dried. The specific nature of the coating substances forms no limitation on this invention, and they may consistof or comprise any of those materials which have hitherto been used in the art for coating paper and board. Among other binders which may be employed in accordance with the above entitled patent for the coating composition, is starch. Very beautiful sheets having a soft matte finish are produced, but unless the aforesaid starchy binder is insolubilized, the sheets will not be resistant to Wet rubbing.

In a copending application of Harry C. Fisher and Brainard E. Sooy, Serial No. 738,722, filed April 1, 1947, and entitled Improvements in Colored Coated Board, now Patent No. 2,515,340, there are described certain procedures for insolubilizing a starchy binder. One of these procedures involves the incorporation of a thermosetting resin in the coating composition, and the use of a catalyst for the resin in the plasticizing solution. A somewhat similar technique, namely the incorporation in the plasticizing solution of the acidic substances serving to insolubilize the casein binder of the coating, is not fully effective in the present invention to attain the purposes set forth above, although it may be employed in connection with the other steps hereinafter set forth for a specific purpose, if desired. To mix the acidic substance with a coating composition containing casein as a' binder also will not attain the objects of this invention because the insolubilizing reaction will be accomplished prior to the time the web reaches the water finishing apparatus, especially since after the coating imprinting step, it is necessary to dry the coating to a relatively hard condition before calendering the web. In addition, mixing the insolubilizing substance with a casein-bearing coating composition prior to the time of the imposition of the composition on the web, results in a premature hardening of the composition in coating reservoirs and in picking of the coating on the imprinting rolls. Since, as will be explained later, We insolubilize surface layers only of the casein coating on the web, an equivalent amount of acidic substance added to the web coating mixture would be insufficient to insolubilize all of it, even if a coating mixture of usableproperties remained, which it does not.

We have found that if the surface or surfaces of a web of paper or board are first plasticized, as hereinabove described, and then, during a persistence of the plasticizing effect, are coated by imprinting upon them a thin layer of coating composition containing, say, mineral coating substances and a" casein binder, the coating being then dried, it becomes possible to calender the coated stock and attain concurrently an insolubilization of the casein binder and waterproofing of the mineral coating and a very high degree of water finish of coated surface by imposing upon the coated surface or surfaces of the web on the finishing calender stacks, and by the usual water boxes, a water solution of alum or other suitable metallic salt or other acidic substances in lieu of the conventional water or water solution of starch or the like. If desired, the acidic substances can be dissolved in the water solution of starch or other compatible substance commonly used to water finish a paper surface on the calender stack and this will not seriously detract from the waterproofness of the insolubilized coating because the actual amount of starch thus imposed on the casein surface is very small. However, any advantage in the obtaining of special surface effects, as in the printing of the coated surface with inks, may be unimportant because similar printed effects can be obtained by changing the composition of the casein-mineral coating mixture.

By a thin imprinted coating, we mean a coating imprinted upon the plasticized surface and having a thickness, after calendering, of substantially a few tenths of one one-thousandth of an inch, or a coating having a dry weight lying in the range between substantially one and four pounds per thousand square feet of paper surface. The lower limit of substantially one pound dry weight represents the smallest amount of coating that can be imposed with complete coverage of the paper surface; the upper limit of substantially four pounds represents the largest amount of coating it is practical to imprint and yet obtain a commercially smooth coated surface. Naturally these quantities of coating are subiect to considerable variations dictated by the quality of paper surface, of coating mixture and of imprinting machinery. The insolubilizing effect described herein is not limited to casein mineral coatings with this coverage and thickness. The preparation of the surface or surfaces of the web by the plasticizing treatment is the thing which enables us to produce a satisfactorily coated web with such very thin layers of coating composition; while the action on the water finishing calender stack hereinafter described is in turn dependent upon the treatment of a very thin layer of casein-bound mineral substance. However. we do not exclude the use of our insolubilizing and finishing process on coatings imposed on a plasticized paper surface by any method whatsoever providing that the coaction of smoothing and insolubilizing of the coating occurs during persistence of the plasticizing effect.

Referring to the drawing, we have shown a web 1 as it comes from the end of the paperboard machine passing through a breaker stack of calender rolls 2. A plasticizing solution, as hereinabove described, is applied to a surface or both surfaces of the web 1 by means of Water boxes 3 and 4. A greater number of water boxes may be employed if desired. The mechanical work exerted on the surfaces of the web 1 by the calender rolls 2 in the presence of the plasticizing solution smooths and plasticizes surface portions of the web fitting them for the smooth reception of the imprinted coating. The coating is imprinted upon the web by means of imprinting rollers 6 or 7, or both, being fed with the coating composition 8 or 9 by transfer rolls 10 or 11. As above indicated, the imprinting is accomplished during a persistence of the plasticizing effect and usually before the complete elimination by evaporation of the water vehicle of the plasticizing solution. The coated web, passing beyond the imprinting rollers, is preferably carried around a series of heated drying rolls 12, 13 and 14 (which may be multiplied as desired to produce an adequate drying effect), and then is passed through a second set of calendering rolls 15, which is a water finishing calender stack and is provided with one or more water boxes 16 and 17. Additional calender stacks may be used.

It will be understood that the coated web reaching the water finishing calender stack is characterized by a casein binder or protein binder or binder component which is not insolubilized and which, while in a relatively dry condition, is capable of being softened by water. If in order to secure a water finish water alone were applied to the coated surface portions of the web on the calender stack 15 by means of the water boxes 16 and 17, no useful result would be attained since the binder of the coating would be so softened by the water that the coating would pick off on the calender rolls, fouling them, redepositing the removed coating sporadically on the sheet surfaces and to all intents and purposes ruining the coated paper or board. We have discovered, however, that when, instead of water in the boxes 16 and 17, we employ a solution of acidic substance capable of insolubilizing the casein, a peculiar action results. Although a softening of the binder occurs due to the action of the water vehicle of the acidic solution, as demonstrated by the actual smoothed surface of coating produced, the insolubilizing proceeds at such a rate relative to the softening action as to prevent picking. The surface finishing action depends upon the employment of a layer of coating composition that is not too thick because, the insolubilizing effect being largely confined to the outer portion of the layer of coating, too thick a layer of coating leaves too much coating substance interior of the layer not insolubilized and still susceptible to softening by water. As hereinafter disclosed, the additional use of acidic substance in the plasticizing solution employed on the breaker stack of calenders insolubilizes that portion of the subsequently imposed layer of coating adjacent the paper surfaces, whereby a reduction in the amount of coating layer, not insolubilized, can be obtained. With very thin layers of coating, complete or substantially complete insolubilization of the entire layer of coating results. We have no reason to believe that the insolubilizing action is not coextensive with the actual penetration of the coating. The insolubilizing action does not prevent such a softening of the coating composition as will permit the attainment of a high degree of water finish, although we have found that a completion of the insolubilizing action prior to the water finishing treatment will not give a good water finish.

In the water boxes 16 and 17, we prefer to employ substantially a 4% solution of dry alum, A12(SO4)3.18H2O, or a solution in the range substantially from 2% to 6%. Expressed otherwise, the acidic solution employed should have strength substantially in the range from one-fifth to three-fifths normal. Considerable variation in the strength of the solution is permissible. The limitations are, first, that in applying the solution by means of the water boxes on the calender stack and in the light of the squeegeeing action of the calender rolls, each water box is capable of treating the coating with only a limited quantity of solution. While, depending upon the number of rolls in the calender stack, a plurality of boxes may be employed to treat the same coated surface of the web, there is a limit to the number of water boxes which can be so used (the web may become too dense or too damp), and the strength of the solution should be such that the web will be treated with an adequate amount of acidic substance to produce the desired resistance to wet rubbing. An excess amount of acidic substance does not seem to be harmful per se, since the solution is applied substantially at the nip of the rolls which do the calendermg, and the softening action proceeds concurrently with the insolubilizing action. We have not encountered a condition in which an excess of the acidic substance produces so rapid an insolubilizing effect as to preclude the attainment of the objects of this invention. On the other hand, an excess of acidic substance over that required to produce wet rubbing resistance is undesirable from the cost standpoint, and in the case of acidic substances which have an initial odor but lose it upon drying, is undesirable for other reasons.

The imposition of acidic solution on the paper surface in such a way that it continually comes into contact with the highly finished steel rolls of the calender stack is repugnant to the established principles of the finishing art, since such solutions tend to attack, roughen and etch the roll surfaces. With solutions in the negihborhood of 4% strength of alum or its equivalent, the corrosion has not been prohibitive. Attack on the rolls may be further minimized by chromium plating them. Chemical buffers may also be employed in our invention as ingredients of the water finishing and insolubilizing solutions. The choice of chemical buffers is somewhat limited in the present invention being confined to substances which will' because they color it yellow. We have successfully employed formalin, adding this substance in the amount ofone quart of commercial formalin to 250 gallons of insolubiliziiig" salaries. Th rerarsiia appears to have a n retarding chemical attack on the We do not regard the specific nature of the acidic substance as a limitation on the invention.

In the practice of our invention the mineral coating mixture comprises the casein adhesive dissolved in water and thoroughly mixed with finely such as clay. The mixture may contain minor proportions of other substances used to improve the coating. making up a mixture granulated casein may be soaked in water, then brought into solution by the use of an alkaline substance such as sodium carbonate, sodium hydroxide, borax, ammonia and the like with gentle heat. amount of preservative, such as a chlorinated phenol, may be added to prevent the spoiling of the casein. Instead of or in addition to casein, other proteinaceous materials to use casein alone or as the greater part of the adhesive. A casein solution of pourable consistency is preferably first made up, and will usually contain around of casein by weight. This is mixed with the mineral matter of the quantity of casein in the final casein mineral matter mix ture will usually amount to substantially 14% to 16% of the dry mineral solids. These proportions may be departed from, as for example if greater or less porosity of coating is desired.

As indicated,

which the casein is used as a binder does not form a limimineral or other pigments Where a tinted or colored coating is desired.

The coating mixture, formed initially in a blade-type mixing device, usually can be improved by passing it through a disperser, ink grinding mill or the like. A small amount of dispersing and liquefying agent, such as tetrasodium pyrophosphate, sodium silicate, etc., may be incorporated into the mixture. Likewise, a small amount of bodying agent, such as sodium carboxymethyl cellulose, may be dissolved in the solution to improve the body and texture of the mixture. Yet again, a quantity of plasticizing substance, such as ammonium stearate, may be added to improve smoothness, flexibility and slip of the coating on the board.

It has hitherto been proposed in casein coating compositions to replace up to about one-half of the casein with the solids from asynthetic latex comprising an aqueous dispersion of a styrene-butadiene copolymer, vinyl resins, and the like, to give a rubbery feel, a smoother surface and improved folding characteristics. Such a resin will itself increase the water resistance of the clay coating mixture, but even with such compositions the calender treatment with a water solution of acidic substance, e. g. alum, will exercise its waterproofing and waterfinishing elfect on the coated surface. In the claims which follow, the term casein binder is to be understood as relating to coatin compositions of which so much of the adhesive or binder 1s casein or its proteinaceous attain water resistance and wet rubbing resistance.

Since the insolubilizing action of an acidic solution applied by means of the water boxes on a finishing stack of calenders extends inwardly from the outer surface of the coating layer and is largely concentrated to that outer surface of the coating, an enhanced degree of moisture aridwet rubbing resistance in the easing can rreqeenay be attainecfiby additionally insolubilizing the under portion of the coating layer, as byincorpor'ating'f ah acidic substance in the plasticizing solution appliedto the b surface on the breaker stack of calenders. must be taken not to insolubilize the entire coating before it reaches the water finish calender sta'ck, Weoldin'ar'ily use" the same concentration and kind of acid substance in the plasticizing solution as in the finishing solution but, of course, can vary it if desired for some particular purpose.

Our invention is not limited to white or to any particular tint or color of coated paper or board. Various colored effects may be attained, by incorsolution By the procedure herein outlined, we produce coated paperboard having a hard, glossy, smoothed, even surface finish excellently adapted to receive printing and in the case of the board, excellent for the conversion into cartons and containers.

Having thus described our invention in an exemplary embodiment, what We claim as new and desire to secure by Letters Patent is:

1. A process of manufacturing a web of coated paper insolubilizing substance for the said protein.

e process of claim 1 wherein the thickness of ndered layer of dried insolubilized coating composition is substantially less than one one-thousandth of an inch.

3. The

dried is a solution of dried crystalline aluminum sulphate having a strength between 2 and 6 per cent by weight. 4. The process of claim 1 wherein the protein binder is casein.

5. The process of claim 1 wherein the binder comprises a blend of protein and resinous substance.

process of claim said agent is a strenlgth between substantially one-fifth and three-fifths References Cited in the file of this patent norma 10. The process claimed in claim 8 in which a water UNITED STATES PATENTS compatible coloring material forms an ingredient of one Number Name Date 1 at least of the treatment substances applied to said web. 5 1,997,984 Swan Apr. 16, 1935 i 11. The process claimed in claim 7 in which an organic 2,291,616 Fletcher Aug. 4, 1942 substance of film forming nature is included in the acidic 2,293,278 Cates Aug. 18, 1942 insolubilizing solution applied to the outer coated surface 2,304,818 Grupe Dec. 15, 1942 of the web. 2,364,505 Bennett Dec. 5, 1944 10 2,419,207 Fisher Apr. 22, 1947 2,515,340 Fisher July 18, 1950 2,611,717 Sooy Sept. 23, 1952 

1. A PROCESS OF MANUFACTURING A WEB OF COATED PAPER OR BOARD WHICH COMPRISES APPLYING TO THE SURFACE OF THE WEB A PLASTICIZABLE FILM FORMING WATER-BORNE SUBSTANCE CHEMICALLY INERT TO THE FIBERS OF THE WEB, AND PASSING THE WEB THROUGH A BREAKER STACK OF CALENDER ROLLS TO PLASTICALLY SMOOTH THE TREATED SURFACE, AND DURING A PERSISTENCE OF THE SAID PLASTICIZING EFFECT, IMPRINTING THEREON A THIN LAYER OF COATING COMPOSITION CONSISTING ESSENTIALLY OF MINERAL SUBSTANCE AND PROTEIN BINDER IN WATER, DRYING THE WEB, AND SUBJECTING IT TO A WATER FINISHING TREATMENT BY PASSING IT THROUGH A STACK OF METALLIC FINISHING CALENDER ROLLS AND APPLYING TO THE TREATED SURFACE OF THE WEB WHILE PASSING THROUGH SAID STACK A WATER SOLUTION OF AN INSOLUBILIZING SUBSTANCE FOR THE SAID PROTEIN. 